A number of techniques have been proposed or suggested for improving the speed and accuracy of digital communications. In particular, a number of techniques have been developed to compensate for the distortion present in many digital communications channels. For example, a number of techniques have been developed to compensate for multi-path fading, whereby multiple copies of an information signal reach the receiver over multiple paths with different time delays, amplitudes, and phases due to scattering and reflection. As the multiple copies of the information signal destructively interfere with each other, the level of the received signal falls. For a detailed discussion of such multi-path fading compensation techniques, see, for example, Turin, G. L. et al. “On Optimal Diversity Reception,” IRE Trans. Inform. Theory, vol. IT-7, pp. 154-166, July 1961, incorporated by reference herein. Generally, if the multi-path fading causes the signal-to-noise ratio (SNR) of the received signal to fall below a usable threshold level, the channel is said to be in a deep fade. For channels with slow fading characteristics, namely, channels whose characteristics vary slowly relative to the data transmission rate, a deep fade can result in long bursts of bit errors.
Antenna diversity is often used to reduce the effects of multi-path fading on channels with slow fading characteristics. The various signals received on each of the multiple antennas can be combined using various techniques, including equal gain combining, maximum ratio combining or by selection diversity techniques. Equal gain combining techniques add each received signal. Maximum ration combining techniques weight each received signal based on a measured power level, thereby emphasizing the stronger signal, before adding each scaled signal. Selection diversity techniques compare the received signal strength from each antenna and select one received signal for processing.
While conventional antenna diversity techniques generally contemplate combining the various received signals in the time domain, it is the frequency response of the original signal that is physically transmitted to the receiver in an orthogonal frequency division multiplexing (OFDM) communication scheme. Thus, combining the diversity branches in the frequency domain will have a different effect on the original data stream than the impact of standard diversity techniques. A need therefore exists for a method and apparatus for combining in the frequency domain the various signals received on each of the multiple antennas in an OFDM communication system.